EP0404621A1 - Attitude control system using a supraconductive magnetic coil - Google Patents

Abstract

The satellite carries a solar generator (2) which can be deployed and oriented with respect to the satellite body (1) and which is intended to be maintained permanently facing the sun, and a device (4) for stabilizing the attitude, characterized in that this attitude- stabilizing device consists of at least:… &cirf& a closed loop (7) of superconducting material carried by a support (6) connected to the solar generator and placed on a face of this support which is oriented approximately in a direction opposite to that of the sun;… &cirf& a secondary loop (9) parallel to the closed loop, of approximately the same shape and size as the closed loop and carried by the support near the closed loop;… &cirf& a resistive circuit (10) carried by this support near at least a part of the closed loop;… &cirf& a selective electrical supply circuit (11) connected electrically to the secondary loop and to the resistive circuit.

Description

The invention relates to an attitude control and stabilization system for satellites.

Whatever its orbit, a satellite is subject to slowly varying external disturbing torques linked to its environment, the main causes of which are:
- atmospheric drag (especially in the case of low orbit),
- solar radiation pressure,
- the terrestrial gravity gradient,
- the Earth's magnetic field.

These couples appear generally due to the asymmetry of the satellite (on the mechanical or geometric level, or that of the mechanical or optical properties of the materials used).

In the case of a satellite stabilized by rotation, the action of these disturbances is slowed down thanks to the gyroscopic rigidity offered by the rotation of the body of the satellite.

In the case of a three-axis stabilized satellite, such ridiculousness is generally created artificially by means of inertia wheels. The effect of external torques - variation of the angular momentum vector - can be canceled by appropriately controlling the speed of the wheels (creation of a speed difference). When the speed limit is reached, the "desaturation" of the wheels (differential speed) is carried out using attitude control actuators: nozzles, solar sails or magnetic coils in general.

The main drawback of the use of nozzles leads to brutal attitude disturbances, to be amortized over a long period of time, leading to mission interruptions, in particular for earth observation.

The use of magnetic coils allows continuous compensation. However, the low value of the Earth's magnetic field (around 10⁻⁷ tesla) and the level of the couples to compensate (10⁻⁵ to 10⁻⁴Nm depending on the asymmetry of the satellite) lead to typical performances of 100 to 1000 amperes.tours.m² and therefore to masses generally large magnetic coils (10 to 20 kg or more). The use of magnetic coils is thus generally limited to control on a single axis of the satellite, for the desaturation of the flywheels.

In addition, certain satellite configurations may exhibit strong asymmetry. For example, many earth observation missions require cooling of the focal plane of the infrared instrument, with a view to satisfactory performance in the IR (infrared) wavelengths. The required temperatures (<100 ° K) are generally obtained by means of passive radiators, located on the non-sunny north or south faces of the satellite (we assume the geostationary orbit).

In the case of three-axis stabilization, these faces are generally used to locate the two wings of an orientable solar generator. However, such appendages are incompatible with the correct operation of a passive radiator which requires a perfectly clear field of view towards space cold.

This leads to resorting, for example, to an asymmetrical configuration, with a single-wing solar generator, located on the face opposite to the cooler (s). Compensation for the high solar radiation pressure torque thus created generally requires the installation of a "solar sail" at the end of a very long mast, on the face opposite to the wing of the solar generator. The level (around 10⁻⁴Nm) and the orientation of this torque make the magnetic coils hardly suited to this scenario.

This solution involves, in addition to its mass, the following drawbacks:
- a decrease in the performance of the cooling system dissement, since the sail is necessarily in the field of view of the latter,
- the deployment of the sail constitutes a single point of failure, which we seek to avoid for reasons of reliability,
- the impossibility of modulating the couple created opposite the pressure couple of solar radiation, while the latter undergoes a seasonal variation (due to the inclination of the plane of the Earth's orbit).

The invention aims to overcome the aforementioned drawbacks by proposing a satellite with solar generator equipped with an attitude compensation device which is light, reliable, compact and energy efficient and which is adapted to effectively attenuate the disturbing couples in particular due at the pressure of solar radiation.

The invention thus proposes a satellite comprising a deployable solar generator orientable with respect to the satellite body and intended to be permanently maintained facing the Sun, as well as an attitude stabilization device, characterized in that this device attitude stabilization includes at least:
- a closed loop of superconductive material carried by a support linked to the solar generator, and arranged on one face of this support which is substantially oriented away from the Sun;
- A secondary loop parallel to the closed loop, substantially of the same shape and dimensions as the closed loop, carried by this support near this loop;
- a resistive circuit carried by this support near at least part of the closed loop; and
- an electric selective supply circuit electrically connected to the secondary loop and to the resistive circuit.

According to preferred arrangements of the invention:
- the closed loop of superconductive material is in a plane parallel to the panels of the solar generator;
- the support is either an additional panel mechanically linked to the solar generator, or a panel thereof (carrying solar cells);
- this additional panel is articulated on the end of the solar generator which is furthest from the body of the satellite;
the closed loop of superconductive material is formed of a substantially circular ribbon or else by the deposition of thin layers of superconductive material on a substrate, or the like;
- The closed loop is carried by a frame mechanically connected to the support by means of thermally insulating material;
- The secondary loop is arranged on the support opposite this loop;
- the resistive circuit and the secondary loop are electrically supplied by the solar generator;
- the solar generator has a high asymmetry, for example with a single wing.

The solution proposed by the invention is therefore based on:
- the use of a current loop located on the wing of the solar generator and interacting with the earth's magnetic field, so as to create a torque that is substantially equal and directly opposite to the torque of solar pressure,
- the installation of this circuit on the "shadow" side (not exposed to solar radiation) of the solar generator (solar panel or one of its appendages) and at a suitable distance from the body of the satellite, so as to obtain a temperature compatible with the use of superconductive materials for current circulation.

This attitude stabilization device essentially consists of the following elements:
- a current loop of superconductive material, for example of the YBa₂Cu₃O₇, Bi₂ Sr₂ Ca₂ Cu₃O₁₀ or Bi₂ Sr₂ Ca Cu₉O₈ type ... presenting a critical temperature compatible with the temperature imposed by the proximity of the surrounding elements (solar generator, body of the satellite) . The characteristics of this loop are such that its magnetic moment interacts with the earth's magnetic field by creating a torque that is substantially equal to and opposite to the value of the pressure couple of solar radiation exerted on the satellite. The external shape and the section of the circuit are arbitrary, provided that the preceding condition, at least approximately, is fulfilled;
- a support, receiving in addition to the previous superconductive circuit, a resistive circuit made of conventional resistive material (nickel-chromium, graphite or other) intended to provide heating of limited duration of at least part of the superconductive loop (one to a few minutes ) of the first circuit, causing its transition to the non-superconductive state;
- an auxiliary current loop of conventional conductive material (for example an aluminum winding or the like), adjacent and capable of being structurally linked to the preceding ones. The latter is intended:
. to the initializations of the current in the superconductive loop,
. to replace the main loop in the event of its breakdown.

The characteristics of this auxiliary loop are such that it can generate a magnetic flux equal to that which should prevail in the main loop under nominal conditions, the latter being obtained by "transfer" from one loop to the other (conservation of the magnetic flux ). The heat dissipation due to this circuit must also remain compatible with the temperature requirements of the main loop.

The entire device can be installed on a solar panel of the solar generator or on an appendage of the latter.

In the stored solar generator configuration, the device can be either outside (space side) or inside the solar generator. The second solution has the advantage of authorizing an operation of the solar generator "without deployment" in transfer orbit.

• 1. Déploiement complet du générateur solaire en orbite. Le dispositif se stabilise thermique­ment à une température correspondant à un état supraconducteur.
• 2. Réchauffage de la boucle supraconductrice de façon à provoquer la transition de la boucle à l'état non supraconducteur (T > Tc).
• 3. Alimentation de la boucle auxiliaire de façon à générer le flux nominal dans le dispositif. La puissance nécessaire à cette initialisation est assurée par le générateur solaire, ainsi que celle nécessaire au réchauffage.
• 4. Arrêt du réchauffage.
• 5. Arrêt de l'alimentation de la boucle auxi­liaire.
• 6. Fonctionnement nominal de la boucle supracon­ductrice, le couple produit équilibrant sensiblement le couple de pression solaire.
• 7. Alimentation contrôlée de la boucle auxiliaire de façon à équilibrer les variations saison­nières de la pression solaire (il peut y avoir plusieurs boucles supraconductrices et plusieurs boucles auxiliaires dont un nombre variable est actif à un moment donné).

In operation, the sequence of the main operations is as follows:

• 1. Full deployment of the solar generator in orbit. The device stabilizes thermally at a temperature corresponding to a superconductive state.
• 2. Heating of the superconductive loop so as to cause the transition of the loop to the non-superconductive state (T> Tc).
• 3. Supply of the auxiliary loop so as to generate the nominal flux in the device. The power required for this initialization is provided by the solar generator, as well as that required for heating.
• 4. Heating stops.
• 5. Power off of the auxiliary loop.
• 6. Nominal operation of the superconducting loop, the torque produced appreciably balancing the solar pressure torque.
• 7. Controlled supply of the auxiliary loop so as to balance the seasonal variations in solar pressure (there may be several superconductive loops and several auxiliary loops of which a variable number is active at a given time).

• - la figure 1 est une vue schématique d'un satellite conforme à l'invention comportant une boucle supraconductrice pour sa stabilisation en attitude. Ce générateur solaire étant vu de dos, et
• - la figure 2 est une vue en coupe selon la ligne 2-2 de la figure 1 du dispositif de stabilisation en attitude de la figure 1.

Objects, characteristics and advantages of the invention appear from the description which follows, given to by way of nonlimiting example, with reference to the appended drawings in which:

• - Figure 1 is a schematic view of a satellite according to the invention comprising a superconductive loop for its stabilization in attitude. This solar generator being seen from the back, and
• - Figure 2 is a sectional view along line 2-2 of Figure 1 of the attitude stabilization device of Figure 1.

FIG. 1 represents an example of configuration of a satellite according to the invention.

It mainly comprises a satellite body 1 equipped with a solar generator 2 deployable between a folded configuration (launch phase) and a deployed service configuration in which it is connected to the body 1 by an arm 3. This solar generator is orientable vis opposite the satellite body and is kept, by known conventional means, permanently facing the sun.

The satellite also includes an attitude stabilization device 4 carried by this solar generator.

In the example considered here, the satellite is in geostationary orbit and has an asymmetrical configuration (single-wing solar generator facing north) giving rise to a disturbing torque T due to the pressure of solar radiation on this solar generator. An average value of the order of 10⁻⁴ Newton.meter will be considered here for this couple T.

, soit environ 1.000 ampères. m², pour une valeur du champ terrestre de l'ordre de 10⁻⁷ Tesla à cette position spatiale.The magnetic moment necessary for the compensation of this couple is worth M = $\frac{\text{T}}{\text{B}}$

, or about 1,000 amps. m², for a value of the Earth's field of the order of 10⁻⁷ Tesla at this spatial position.

The typical dimensions of the solar generator considered here (length 5 to 10 m, width 1 to 2.5 m) and the constraints of storage under cover at launch of the staellite, in practice allow the implantation of the compensation loop 4 on a shutter of the solar generator or, as is the case here, on an additional shutter of dimensions typically equal to 1.5 meters per side (square), here articulated like the other shutters of the solar generator, at the free end of the latter.

This additional flap or panel, on which the compensation system 4 rests, comprises a support panel 6, a closed superconductive loop 7, a support frame 8, an auxiliary loop 9, a resistive circuit 10 and a control circuit 11.

The support panel 6 can be made of composite material ("Nida" fiberglass sandwich type "NOMEX" with carbon soles for example); the typical thickness is approximately 1.5 cm for the lateral dimensions considered.

This panel is coated with a material 12 with high reflectivity (SSM or OSR, English abbreviations for "Secondary Surface Mirror" or "Optical Surface Reflector") on the face exposed to solar radiation for a minimum temperature.

The closed loop 7 is made of YBa₂ Cu₃ O₇ type superconductive material, forming the main current loop of the compensation device. Other superconductive materials can also be used: Bi₂ Sr₂ CaCu₂O₈, Th₂Ba₂ CaCu₂O₈, Bi₂ Sr₂ Ca₂ Cu₂ O₁₀ ..... We will consider here a circular external shape, corresponding to the use of the material in the form of ribbon. Different shapes are also conceivable (rectangular (with favorable rounded angles at high current densities) ...) in the case of an embodiment of the superconductive material in the form of a deposit of thin layer on a substrate.

Typical ribbon dimensions are:
- outside diameter of the loop: about 1.5 meters
- width: about 1 cm
- thickness: about 10 to 100 µm.

Such values allow current densities of 10⁴ to 10⁵ amperes / cm² and are therefore compatible with the intensity necessary to obtain the desired magnetic moment, i.e. here:

The frame 8 supports the previous circuit and is of the "crown" type. This frame can be made of composite material like the panel 6 and has an adequate coating, intended to maintain the temperature of the loop 7 at a level corresponding to its superconductive state (here of the order of 75 ° K taking into account a suitable margin). In principle, black paint will be used for this purpose on the side of the loop 7 side and a multi-layer thermally insulating coating on the other side (side of the panel 6).

The frame 8 is mechanically connected to the support panel 6 by means of supports 13 made of material with very low thermal conductivity (carbon fiber for example) of a height ensuring sufficient spacing between the panel 6 and the frame 8 (typically 10 cm) .

The auxiliary circuit 9 is made of conductive material (aluminum for example); it is intended for the initialization of the current in the main loop 7. Its external dimensions are substantially the same as those of the loop (equivalent circular sections, neighboring diameters) and its installation is carried out on the opposite face (exposed to the sun) for a minimal thermal impact on the loop 7 when the latter is in the superconductive state and current flows in the loop 9.

This auxiliary circuit or loop is in practice a winding with turns parallel to the loop 7.

When the system is initialized, this auxiliary loop 9 is supplied with current by means of the solar generator (control on board the satellite), for the time necessary (of the order of a few minutes) for the initialization operations. The current to flow is such that the flow induced in the main loop 7 corresponds to the desired current intensity in this loop. Given the strong coupling between the two loops, a value close to Ampere.tours distributed over n turns of current is necessary, i.e. around 600 Ampere.tours here.

The number of turns n is such that the voltage at the terminals of circuit 9 corresponds at most to that supplied by the solar generator to the satellite. With a voltage of about 50 volts, we obtain for example of the order of 100 to 200 turns of a section of the order of a square millimeter.

The initialization method of loop 7, using an auxiliary loop D described here, is not limiting. One can envisage other devices, based for example on the direct injection of a current into the loop 7, by using a superconductive switch.

The circuit 10 is intended for local heating of the loop 7; it is intended to cancel its superconductive properties of this loop 7 when necessary. This circuit is made by means of a resistor, a few centimeters in length, made of resistive material (such as Nickel Chromium or graphite for example), powered by the solar generator.

The control circuit shown diagrammatically at 11 is connected to the electrical part of the solar generator (or to batteries) to the auxiliary loop 9 and to the resistive circuit 10. It is used to trigger and interrupt the electrical supply of the elements 9 and 10.

How the whole system works is carried out as previously indicated.

The modulation of the compensation torque so as to compensate for the seasonal variations in the torque of solar radiation pressure (of the order of 10%, due to the 23 ° 5 tilt of the Earth's orbit), or any other disruptive of a different nature, possibly, can be obtained:
- By an operation of the loop 7 controlled in all or nothing: this loop 7 is dimensioned so as to compensate for the maximum value of the disturbing torque; and its operation is periodically interrupted according to a predetermined control law (or determinable on the ground or on board) adapted to follow on average the variations in disturbing torque. There is no limit to the number of interruptions / resets;
- By a device for controlling the level of the total magnetic moment of the system described above;
- by the use of a secondary loop decoupled from the previous ones, the intensity of the current flowing therein is subject to following only seasonal or other variations;
- By the implementation of several independent sets 4, of advantageously different powers; by the appropriate choice of those of these sets which one activates or not at a given moment, one can vary the total compensating torque and follow in stages the variations of the disturbing torque (for example there can be 3 sets including two sets of lower power than the first).

It goes without saying that the foregoing description has been offered only by way of nonlimiting example and that numerous variants can be proposed by those skilled in the art without departing from the scope of the invention.

Claims (10)

1. Satellite comprising a deployable solar generator (2) orientable with respect to the body (1) of the satellite and intended to be permanently maintained facing the Sun, as well as a device (4) for attitude stabilization, characterized in what this attitude stabilization device comprises at least:
- a closed loop (7) of superconductive material carried by a support linked (6) to the solar generator, and disposed on one face of this support which is substantially oriented opposite the Sun;
- A secondary loop (9) parallel to the closed loop, substantially of the same shape and dimensions as the closed loop, carried by this support near this loop;
- a resistive circuit (10) carried by this support near at least part of the closed loop; and
- an electric circuit (11) of selective supply electrically connected to the secondary loop and to the resistive circuit. 2. Satellite according to claim 1, characterized in that the closed loop (7) of superconductive material is in a plane parallel to the panels of the solar generator. 3. Satellite according to claim 1 or claim 2, characterized in that the support (6) is an additional panel mechanically linked to the solar generator. 4. Satellite according to claim 3, characterized in that this additional panel is articulated on the end of the solar generator which is furthest from the body of the satellite. 5. Satellite according to any one of claims 1 to 4, characterized in that the closed loop (7) of superconductive material is formed of a substantially circular ribbon. 6. Satellite according to any one of claims 1 to 4, characterized in that the closed loop consists of a deposit of thin layers of superconductive material on a substrate. 7. Satellite according to any one of claims 1 to 6, characterized in that the closed loop is carried by a frame (8) mechanically connected to the support by means (13) of thermally insulating material. 8. Satellite according to any one of claims 1 to 7, characterized in that the secondary loop (9) is arranged on the support opposite this loop. 9. Satellite according to any one of claims 1 to 8, characterized in that the resistive circuit (10) and the secondary loop (9) are electrically supplied by the solar generator. 10. Satellite according to any one of claims 1 to 9, characterized in that the solar generator is single wing.

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